Optical fibers which are made of silica glass and which include cores doped with an alkali metal element are known (PTLs 1 to 9). It is said that in the case where the core portion of an optical fiber preform is doped with the alkali metal element, the viscosity of the core portion can be reduced during drawing of the optical fiber preform, relaxation of a network structure of the silica glass proceeds and, thereby, an attenuation of the optical fiber can be reduced.
As a method for doping the silica glass with the alkali metal element, the diffusion method is known (PTLs 1 and 2). In the diffusion method, a glass pipe is heated from the outside or plasma is generated in the glass pipe while a vapor of the alkali metal element or an alkali metal salt, which serves as a source material, is introduced into the glass pipe. The inside surface of the glass pipe is thereby doped with the alkali metal element by diffusion.
After the vicinity of the inside surface of the glass pipe is doped with the alkali metal element as described above, the diameter of the resulting glass pipe is reduced by heating. After the reduction in diameter, some thickness of the inside surface of the glass pipe is etched for the purpose of removing transition metal elements, e.g., Ni and Fe, which are added at the same time with addition of the alkali metal element. The alkali metal element diffuses faster than the transition metal element. Therefore, even when some thickness of glass surface is etched to remove the transition metal element, it is possible to allow the alkali metal element to remain. After the etching, the glass pipe is heated and collapsed, so that an alkali metal element-doped core rod is produced. A cladding portion having a refractive index smaller than that of the core portion including the resulting rod is synthesized on the outside of the alkali metal element-doped core rod, so that an optical fiber preform is produced. Then, an optical fiber can be produced by drawing the resulting optical fiber preform.